Light emitting device including planarization layer, method of manufacturing the light emitting device, and display apparatus including the light emitting device

ABSTRACT

Provided a light emitting device including a reflective layer including a plurality of nanostructures that are periodically two-dimensionally arranged, a planarization layer disposed on the reflective layer, a first electrode disposed on the planarization layer, an organic emission layer disposed on the first electrode, and a second electrode disposed on the organic emission layer, wherein the planarization layer includes a conductive material that is transparent with respect to light emitted by the organic emission layer, and wherein the planarization layer is disposed on upper surfaces of the plurality of nanostructures such that an air gap is provided between adjacent nanostructures of the plurality of nanostructures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 U.S.C. § 119to Korean Patent Application No. 10-2020-0087650, filed on Jul. 15,2020, in the Korean Intellectual Property Office, the disclosure ofwhich is incorporated by reference herein in its entirety.

BACKGROUND 1. Field

Example embodiments of the present disclosure relate to a light emittingdevice, a method of manufacturing the light emitting device, and adisplay apparatus including the light emitting device, and moreparticularly to, an organic light emitting device that has high colorpurity without using a color filter and is more easily manufactured, amethod of manufacturing the organic light emitting device, and anorganic light emitting display apparatus.

2. Description of Related Art

An organic light emitting device (OLED) is a display apparatus thatforms an image via light emission according to a combination of holessupplied from an anode and electrons supplied from a cathode in anorganic emission layer. The OLED has excellent display characteristicssuch as a wide viewing angle, a fast response speed, a small thickness,a low manufacturing cost, and a high contrast.

Further, the OLED may emit a wanted color according to selection of anappropriate material as a material of the organic emission layer.According to this principle, it may be possible to manufacture a colordisplay apparatus by using the OLED. For example, an organic emissionlayer of a blue pixel may include an organic material that generatesblue light, an organic emission layer of a green pixel may include anorganic material that generates green light, and an organic emissionlayer of a red pixel may include an organic material that generates redlight. Also, a white OLED may be manufactured by arranging a pluralityof organic materials which respectively generate blue light, greenlight, and red light in one organic emission layer or by arranging pairsof two or more kinds of organic materials in a complementaryrelationship with each other.

SUMMARY

One or more example embodiments provide a light emitting device, amethod of manufacturing the light emitting device, and a displayapparatus including the light emitting device, and more particularly anorganic light emitting device that has high color purity without using acolor filter and is more easily manufactured, a method of manufacturingthe organic light emitting device, and an organic light emitting displayapparatus.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the example embodiments of the disclosure.

According to an aspect of an example embodiment, there is provided alight emitting device including a reflective layer including a pluralityof nanostructures that are periodically two-dimensionally arranged, aplanarization layer disposed on the reflective layer, a first electrodedisposed on the planarization layer, an organic emission layer disposedon the first electrode, and a second electrode disposed on the organicemission layer, wherein the planarization layer includes a conductivematerial that is transparent with respect to light emitted by theorganic emission layer, and wherein the planarization layer is disposedon upper surfaces of the plurality of nanostructures such that an airgap is provided between adjacent nanostructures of the plurality ofnanostructures.

The planarization layer may further include an organic-inorganic hybridlayer in which reduced graphene oxide is dispersed.

The organic-inorganic hybrid layer may include an organic siliconcompound.

A proportion of the reduced graphene oxide dispersed in theplanarization layer may be in a range of 1.5 wt % to 15 wt %.

A thickness of the planarization layer may be in a range of 10 nm to 50nm.

A surface roughness of the planarization layer may be less than 1 nmroot-mean-square (RMS).

The surface roughness of the planarization layer may be in a range of0.3 nm RMS to 0.5 nm RMS.

The first electrode may be a transparent electrode and the secondelectrode may be a semi-transmissive electrode that is configured toreflect part of light and transmit a remaining part of the light.

The second electrode may include a reflective metal, and a thickness ofthe second electrode may be in a range of 10 nm to 50 nm.

The reflective layer and the second electrode may form a micro cavityhaving a resonance wavelength.

The resonance wavelength of the micro cavity is λ, a diameter of each ofthe plurality of nanostructures of the reflective layer, a height ofeach of the plurality of nanostructures, and a period of the pluralityof nanostructures may be determined such that an optical length of themicro cavity satisfies nλ/2, where n is a natural number.

A period of the plurality of nanostructures may be less than theresonance wavelength of the micro cavity.

The reflective layer may include a base, and the plurality ofnanostructures may protrude toward the planarization layer from an uppersurface of the base.

The reflective layer may further include a plurality of recesses thatare recessed from the upper surface of the base and two-dimensionallyprovided.

The reflective layer and the second electrode may form a micro cavityhaving a resonance wavelength, wherein the organic emission layer may beconfigured to emit visible light including light of a first wavelengthand light of a second wavelength, wherein the first wavelength is λ, adiameter of each of the plurality of nanostructures of the reflectivelayer, a height of each of the plurality of nanostructures, and a periodof the plurality of nanostructures may be determined such that anoptical length of the micro cavity satisfies nλ/2, where n is a naturalnumber, and wherein a diameter of each of the plurality of recesses maybe determined such that the plurality of recesses are configured toabsorb the light of the second wavelength.

The reflective layer may include a metal material including silver (Ag),aluminum (Al), gold (Au), nickel (Ni), or an alloy thereof.

According to another aspect of an example embodiment, there is provideda display apparatus including a first pixel configured to emit light ofa first wavelength, and a second pixel configured to emit light of asecond wavelength different from the first wavelength, wherein the firstpixel includes a reflective layer including a plurality ofnanostructures periodically two-dimensionally arranged, a planarizationlayer disposed on the reflective layer, a first electrode disposed onthe planarization layer, an organic emission layer disposed on the firstelectrode, the organic emission layer being configured to emit visiblelight including the light of the first wavelength and the light of thesecond wavelength, and a second electrode disposed on the organicemission layer, wherein the planarization layer includes a conductivematerial that is transparent with respect to light emitted by theorganic emission layer, and wherein the planarization layer is disposedon upper surfaces of the plurality of nanostructures such that an airgap is provided between adjacent nanostructures of the plurality of nanostructures.

The planarization layer may further include an organic-inorganic hybridlayer in which reduced graphene oxide is dispersed.

The organic-inorganic hybrid layer may include an organic siliconcompound.

A proportion of the reduced graphene oxide dispersed in theplanarization layer may be in a range of 1.5 wt % to 15 wt %.

A thickness of the planarization layer may be in a range of 10 nm to 50nm.

A surface roughness of the planarization layer may be less than 1 nmroot-mean-square (RMS).

The surface roughness of the planarization layer may be in a range of0.3 nm RMS to 0.5 nm RMS.

The first electrode may be a transparent electrode and the secondelectrode may be a semi-transmissive electrode that is configured toreflect part of light and transmit a remaining part of the light.

The reflective layer and the second electrode may form a micro cavityhaving a resonance wavelength corresponding to the first wavelength, andwhen the first wavelength is λ, a diameter of each of the plurality ofnanostructures of the reflective layer, a height of each of theplurality of nanostructures, and a period of the plurality ofnanostructures may be determined such that an optical length of themicro cavity satisfies nλ/2, where n is a natural number.

According to yet another aspect of an example embodiment, there isprovided a method of manufacturing a light emitting device, the methodincluding coating a substrate with a mixture of graphene oxide andtetramethyl orthosilicate (TMOS) sol in a solvent, curing the mixturecoated on the substrate, annealing the cured mixture to reduce grapheneoxide to reduced graphene oxide and form a planarization layer,transferring the planarization layer to a reflective layer including aplurality of nanostructures that are periodically two-dimensionallyarranged, disposing a first electrode on the planarization layer,disposing an organic emission layer on the first electrode, anddisposing a second electrode on the organic emission layer, wherein theplanarization layer is conductive and transparent with respect to lightgenerated from the organic emission layer, and wherein the planarizationlayer is disposed on upper surfaces of the plurality of nanostructuressuch that an air gap is provided between adjacent nanostructures of theplurality of nanostructures.

According to yet another aspect of an example embodiment, there isprovided a light emitting device including a reflective layer includinga plurality of nanostructures that are periodically two-dimensionallyprovided, and a plurality of recesses that are periodicallytwo-dimensionally provided, a planarization layer disposed on thereflective layer, a first electrode disposed on the planarization layer,an organic emission layer disposed on the first electrode, and a secondelectrode disposed on the organic emission layer, wherein theplanarization layer includes a conductive material transparent withrespect to light generated from the organic emission layer, and whereinthe planarization layer is disposed on upper surfaces of the pluralityof nanostructures such that an air gap is provided between adjacentnanostructures of the plurality of nanostructures.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects, features, and advantages of certainexample embodiments will be more apparent from the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view schematically showing a structure of alight emitting device according to an example embodiment;

FIG. 2 is a cross-sectional view showing in more detail an examplestructure of an organic emission layer illustrated in FIG. 1 ;

FIG. 3 is a cross-sectional view showing in more detail another examplestructure of an organic emission layer illustrated in FIG. 1 ;

FIG. 4 is a perspective view schematically showing an example structureof a reflective layer illustrated in FIG. 1 ;

FIG. 5 is a structural formula showing an example of graphene oxide;

FIG. 6 is a table showing an example of various combinations ofmaterials for manufacturing a planarization layer;

FIGS. 7A, 7B, 7C, 7D, and 7E are diagrams showing measurement results ofthe surface roughness of a planarization layer according to the contentof graphene oxide;

FIGS. 8A, 8B, 8C, 8D, 8E, and 8F are scanning electron microscope (SEM)images showing planarization layer transferred onto a reflective layer;

FIG. 9 is a graph showing the relationship between the wavelength ofincidence light and the transmittance of a planarization layer withrespect to various contents of graphene oxide;

FIG. 10 is a graph showing changes in the electrical conductivity andthe transmittance of a planarization layer according to the content ofgraphene oxide;

FIG. 11 is a graph showing the current-voltage behavior of aplanarization layer according to the content of graphene oxide;

FIG. 12 is a cross-sectional view schematically showing a structure of alight emitting device according to another example embodiment;

FIG. 13 is a perspective view schematically showing an example structureof a reflective layer illustrated in FIG. 12 ;

FIG. 14 is a plan view schematically showing an example structure of thereflective layer illustrated in FIG. 12 ;

FIG. 15A schematically shows light of a short wavelength flowing into arecess formed in a reflective layer;

FIG. 15B schematically shows light of a long wavelength blocked in thereflective layer in which the recess is formed;

FIG. 16 schematically shows light resonating in a light emitting deviceaccording to an example embodiment;

FIG. 17 is a plan view schematically showing another example structureof the reflective layer shown in FIG. 12 ;

FIG. 18 is a perspective view schematically showing another examplestructure of the reflective layer shown in FIG. 12 ;

FIG. 19 is a cross-sectional view schematically showing a structure of adisplay apparatus according to an example embodiment; and

FIG. 20 is a cross-sectional view schematically showing a structure of adisplay apparatus according to another example embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to example embodiments of which areillustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, the exampleembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein. Accordingly, theexample embodiments are merely described below, by referring to thefigures, to explain aspects. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.Expressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list. For example, the expression, “at leastone of a, b, and c,” should be understood as including only a, only b,only c, both a and b, both a and c, both b and c, or all of a, b, and c.

Hereinafter, with reference to the accompanying drawings, a lightemitting device, a method of manufacturing the light emitting device anda display apparatus including the light emitting device will bedescribed in detail. Like reference numerals refer to like elementsthroughout, and in the drawings, sizes of elements may be exaggeratedfor clarity and convenience of explanation. The example embodimentsdescribed below are merely exemplary, and various modifications may bepossible from the embodiments.

In a layer structure described below, an expression “above” or “on” mayinclude not only “immediately on in a contact manner” but also “on in anon-contact manner”. An expression used in the singular encompasses theexpression of the plural, unless it has a clearly different meaning inthe context. It will be further understood that the terms “comprises”and/or “comprising” used herein specify the presence of stated featuresor elements, but do not preclude the presence or addition of one or moreother features or elements.

The use of “the” and other demonstratives similar thereto may correspondto both a singular form and a plural form. Unless the order ofoperations of a method according to the present disclosure is explicitlymentioned or described otherwise, the operations may be performed in aproper order. The present disclosure is not limited to the order theoperations are mentioned.

The term used in the embodiments such as “unit” or “module” indicates aunit for processing at least one function or operation, and may beimplemented in hardware or software, or in a combination of hardware andsoftware.

The connecting lines, or connectors shown in the various figurespresented are intended to represent functional relationships and/orphysical or logical couplings between the various elements. It should benoted that many alternative or additional functional relationships,physical connections or logical connections may be present in apractical device.

The use of any and all examples, or language provided herein, isintended merely to better illuminate the disclosure and does not pose alimitation on the scope of the disclosure unless otherwise claimed.

FIG. 1 is a cross-sectional view schematically showing a structure of alight emitting device 100 according to an example embodiment. Referringto FIG. 1 , the light emitting device 100 according to an exampleembodiment may include a reflective layer 110 including a plurality ofnanostructures 112 that are periodically two-dimensionally arranged, atransparent planarization layer 120 disposed on the reflective layer110, a first electrode 131 disposed on the planarization layer 120, anorganic emission layer 140 disposed on the first electrode 131, and asecond electrode 132 disposed on the organic emission layer 140. Thelight emitting device 100 may further include a transparent passivationlayer 150 disposed on the second electrode 132 opposite to the organicemission layer 140 to protect the second electrode 132.

The light emitting device 100 may be an organic light emitting diode(OLED). For example, FIG. 2 is a cross-sectional view showing an examplestructure of the organic emission layer 140 illustrated in FIG. 1 inmore detail. Referring to FIG. 2 , the organic emission layer 140 mayinclude a hole injection layer 142 disposed on the planarization layer120, an organic emission material layer 141 disposed on the holeinjection layer 142, and an electron injection layer 143 disposed on theorganic emission material layer 141. In this structure, holes providedthrough the hole injection layer 142 and electrons provided through theelectron injection layer 143 may be combined in the organic emissionmaterial layer 141 to generate light. A wavelength of the generatedlight may be determined according to an energy band gap of a lightemitting material of the organic emission material layer 141.

In addition, the organic emission layer 140 may further include a holetransfer layer 144 disposed between the hole injection layer 142 and theorganic emission material layer 141 in order to further facilitate thetransport of holes. In addition, the organic emission layer 140 mayfurther include an electron transfer layer 145 disposed between theelectron injection layer 143 and the organic emission material layer 141in order to further facilitate the transport of electrons. In addition,the organic emission layer 140 may include various additional layers asnecessary. For example, the organic emission layer 140 may furtherinclude an electron block layer between the hole transfer layer 144 andthe organic emission material layer 141, and may also further include ahole block layer between the organic emission material layer 141 and theelectron transfer layer 145.

The organic emission material layer 141 may be configured to emitvisible light. For example, the organic emission material layer 141 maybe configured to emit light in a wavelength band among a wavelength bandof red light, a wavelength band of green light, and a wavelength band ofblue light. The organic emission material layer 141 may be configured toemit white visible light including red light, green light, and bluelight.

For example, FIG. 3 is a cross-sectional view showing another examplestructure of the organic emission layer 140 illustrated in FIG. 1 inmore detail. Referring to FIG. 3 , the organic emission material layer141 may include a first organic emission material layer 141 a that emitsred light, a second organic emission material layer 141 b that emitsgreen light, and a third organic emission material layer 141 c thatemits blue light between the hole transfer layer 144 and the electrontransfer layer 145. Further, an exciton blocking layer 146 may bedisposed between the first organic emission material layer 141 a and thesecond organic emission material layer 141 b and between the secondorganic emission material layer 141 b and the third organic emissionmaterial layer 141 c. Then, the organic emission layer 140 may emitwhite light. However, the structure of the organic emission layer 140that emits white light is not limited thereto. Instead of including thethree organic emission material layers 141 a, 141 b, and 141 c, theorganic emission layer 140 may also include two organic emissionmaterial layers in complementary color relation to each other.

The first electrode 131 disposed on the lower surface of the organicemission layer 140 may serve as an anode that provides holes. The secondelectrode 132 disposed on the upper surface of the organic emissionlayer 140 may serve as a cathode that provides electrons. To this end,the first electrode 131 may include a material having a relatively highwork function, and the second electrode 132 may include a materialhaving a relatively low work function.

In addition, the first electrode 131 may be a transparent electrodehaving a property of transmitting light (e.g., visible light). Forexample, the first electrode 131 may include transparent conductiveoxide, such as indium tin oxide (ITO), indium zinc oxide (IZO), oraluminum zinc oxide (AZO).

The second electrode 132 may be a semi-transmissive electrode thatreflects part of light and transmits the remaining part of light. Tothis end, the second electrode 132 may include a very thin reflectivemetal. For example, the second electrode 132 may include silver (Ag),aluminum (Al), gold (Au), nickel (Ni), or an alloy thereof, or amultilayer structure of silver (Ag) and magnesium (Mg) or a multilayerstructure of aluminum (Al) and lithium (Li). The entire thickness of thesecond electrode 132 may be about 10 nm to about 50 nm. Because thethickness of the second electrode 132 is very thin, part of light maypass through the reflective metal.

The reflective layer 110 may be configured to reflect light generatedfrom the organic emission layer 140 and transmitted through the firstelectrode 131. In addition, the reflective layer 110 may include aconductive material. To this end, the reflective layer 110 may includesilver (Ag), gold (Au), aluminum (Al), nickel (Ni), or an alloy thereof.However, the reflective layer 110 is not limited thereto, and mayinclude other reflective materials having high reflectivity andconductivity.

The reflective layer 110 may form a micro cavity together with thesecond electrode 132. For example, the micro cavity may be formedbetween the reflective layer 110 and the second electrode 132 of thelight emitting device 100. For example, light generated from the organicemission layer 140 may reciprocate and resonate between the reflectivelayer 110 and the second electrode 132, and then light corresponding tothe resonance wavelength of the micro cavity may be emitted to theoutside through the second electrode 132.

The resonance wavelength of the micro cavity formed between thereflective layer 110 and the second electrode 132 may be determined byan optical length L of the micro cavity. For example, when the resonancewavelength of the micro cavity is λ, the optical length L of the microcavity may be nλ/2, where n is a natural number. The optical length L ofthe micro cavity may be determined as the sum of the optical thicknessof layers forming the micro cavity between the reflective layer 110 andthe second electrode 132, a phase delay by the second electrode 132, anda phase shift (e.g., a phase delay) by the reflective layer 110. Here,the optical thickness of the layers forming the micro cavity between thereflective layer 110 and the second electrode 132 is not a simplephysical thickness, but is the thickness considering the refractiveindex of materials of the layers forming the micro cavity. For example,the optical thickness of the layers forming the micro cavity may be thesum of the optical thickness of the planarization layer 120, the opticalthickness of the first electrode 131, and the optical thickness of theorganic emission layer 140.

According to the example embodiment, the optical length L of or theresonance wavelength of the micro cavity may be adjusted by adjustingonly the phase shift by the reflective layer 110 while fixing theoptical thickness of the layers forming the micro cavity and the phasedelay by the second electrode 132. In order to control the phase shiftby the reflective layer 110, a phase modulation surface may be formed onthe reflective surface of the reflective layer 110 in contact with theplanarization layer 120. The phase modulation surface may include verysmall patterns in the nanoscale. For example, the phase modulationsurface of the reflective layer 110 may have a meta structure in whichnano patterns having a size smaller than the wavelength of visible lightare periodically disposed.

Referring back to FIG. 1 , the reflective layer 110 may include a base111 and the phase modulation surface formed on an upper surface 114 ofthe base 111. The phase modulation surface of the reflective layer 110may include a plurality of nanostructures 112 periodically formed on theupper surface 114 of the base 111. The plurality of nanostructures 112may have a post shape protruding from the upper surface 114 of the base111 toward the planarization layer 120. For example, the plurality ofnanostructures 112 may have a cylindrical shape. The plurality ofnanostructures 112 may be integrally formed with the base 111. Thereflective layer 110 may be disposed such that the upper surfaces of theplurality of nanostructures 112 are in contact with the planarizationlayer 120.

When each of the nanostructures 112 is, for example, a cylinder, theoptical characteristics of the phase modulation surface, for example,the phase delay of reflected light may be determined by a diameter W ofeach of the nanostructures 112, a height H each of the nanostructures112 and a pitch or period P of the plurality of nanostructures 112. Wheneach of the nanostructures 112 is a polygonal column, the opticalcharacteristics of the phase modulation surface may be determined by amaximum width W of each of the nanostructures 112, the height H of eachof the nanostructures 112, and the pitch or the period P of theplurality of nanostructures 112.

The diameter W, the height H, and the period P of the nanostructures 112may be constant with respect to the entire region of the phasemodulation surface. For example, the diameter W of the nanostructure 112may be from about 30 nm to about 250 nm, the height H of thenanostructure 112 may be from about 0 nm to about 150 nm, and the periodP of the plurality of nanostructures 112 may be from about 100 nm toabout 300 nm.

When the size of each of the nanostructures 112 of the phase modulationsurface is smaller than the resonance wavelength as described above, aplurality of nano-light resonance structures may be formed whileincident light resonates in the periphery of the nanostructures 112. Inparticular, in the incident light, an electric field component may notpenetrate into a space 113 between the nanostructures 112, and only amagnetic field component may resonate in the periphery of thenanostructures 112. Accordingly, the plurality of nano-light resonantstructures formed in the space 113 between the nanostructures 112 may bea cylinder type magnetic resonator in which the magnetic field componentof the incident light resonates in the periphery of the nanostructures112. As a result, a phase shift greater than a simple phase shift due toan effective optical distance (H×n) determined by the product of theheight H of the nanostructures 112 and a refractive index n of thenanostructures 112 may occur on the phase modulation surface of thereflective layer 110.

Accordingly, the resonance wavelength of the micro cavity may bedetermined by the diameter W of each of the nanostructures 112 of thephase modulation surface, the height H of each of the nanostructures 112and the period P of the plurality of nanostructures 112. In other words,when the resonance wavelength of the micro cavity is λ, the diameter Wof each of the nanostructures 112 of the phase modulation surface, theheight H of each of the nanostructures 112 and the period P of theplurality of nanostructures 112 of the phase modulation surface may beselected such that the optical length L of the micro cavity satisfiesnλ/2, where n is a natural number.

Then, the resonance wavelength of the micro cavity may more easily matchwith the emitting wavelength or emitting color of the light emittingdevice 100. For example, when the light emitting device 100 is a redlight emitting device, the diameter W of each of the nanostructures 112of the phase modulation surface, the height H of each of thenanostructures 112 and the period P of the plurality of nanostructures112 of the phase modulation surface may be selected such that theresonance wavelength of the micro cavity corresponds to a red wavelengthband. As described above, it may be possible to determine the emittingwavelength of the light emitting device 100 only by the structure of thephase modulation surface of the reflective layer 110.

In order to prevent or reduce the micro cavity from having apolarization dependency, the plurality of nanostructures 112 may beregularly and periodically arranged to have a 4-fold symmetrycharacteristic. When the micro cavity has the polarization dependency,only light of a specific polarization component may resonate, which maydeteriorate the light emitting efficiency of the light emitting device100. For example, FIG. 4 is a perspective view schematically showing anexample structure of the reflective layer 110 illustrated in FIG. 1 .Referring to FIG. 4 , the plurality of nanostructures 112 having acylindrical shape on the upper surface 114 of the base 111 may bearranged two-dimensionally in the shape of a regular square array. InFIG. 4 , although the nanostructure 112 has the cylindrical shape, theshape of the nanostructure 112 is not necessarily limited thereto. Forexample, the nanostructure 112 may be in the shape of a cylindricalcolumn, an elliptical column, a pentagonal or larger polygonal column ora cruciform column.

In addition, in FIG. 4 , the plurality of nanostructures 112 is arrangedin the shape of the regular square array. In this case, intervalsbetween the two adjacent nanostructures 112 in the entire region of aphase modulation surface may be constant. However, if the plurality ofnanostructures 112 has a 4-fold symmetry characteristic, the pluralityof nanostructures 112 may be arranged in any other type of array. Forexample, the plurality of nanostructures 112 may be two-dimensionallyarranged in the shape of a hexagonal array, or two-dimensionallyarranged in the shape of a body-centered square. For example, theplurality of nanostructures 112 may be arranged irregularly. In thiscase, the micro cavity may not also have a polarization dependency.According to another example embodiment, the arrangement of theplurality of nanostructures 112 may be designed differently from the4-fold symmetry such that the light emitting device 100 intentionallyemits only light of a specific polarization component. For example, theplurality of nanostructures 112 may be arranged in a one-dimensionalarray pattern.

The planarization layer 120 may be disposed on the reflective layer 110including the plurality of nanostructures 112 to provide a flat surfacewith respect to the structures on the planarization layer 120.Therefore, the first electrode 131 disposed on the upper surface of theplanarization layer 120 may have a flat lower surface. Then, both thelower and upper surfaces of the first electrode 131 may have flatsurfaces, and the organic emission layer 140 and the second electrode132 disposed thereon may also have flat surfaces. Therefore, the firstelectrode 131 and the second electrode 132 may apply a uniform electricfield to the organic emission layer 140. As a result, the lifespan ofthe organic emission layer 140 and the light emitting device 100 mayincrease.

In addition, the planarization layer 120 may be disposed on the uppersurface of the nanostructure 112 such that an air gap exists between theplurality of nanostructures 112 of the reflective layer 110. Forexample, the planarization layer 120 may be disposed to not fill thespace 113 between the plurality of nanostructures 112. To this end, thelower surface of the planarization layer 120 may also be formed to bealmost flat. In addition, while the planarization layer 120 is disposedon the reflective layer 110, the planarization layer 120 may be formedsufficiently robust such that the surface of the planarization layer 120is not deformed to be concaved toward the space 113 between theplurality of nanostructures 112.

When the space 113 between the plurality of nanostructures 112 is filledwith a metal, semiconductor, or dielectric having a higher refractiveindex than that of air, the optical gap between the plurality ofnanostructures 112 increases by the refractive index of the materialfilled in the space 113 between the plurality of nanostructures 112.Then, in order to form the period of the plurality of nanostructures 112to correspond to the designed period, the physical distance between theplurality of nanostructures 112 may need to be reduced in considerationof the refractive index of the material filled in the space 113 betweenthe plurality of nanostructures 112. For example, when an air gap existsbetween the plurality of nanostructures 112 and the physical period ofthe plurality of nanostructures 112 is about 150 nm, the physical periodof the plurality of nanostructures 112 may be less than 100 nm when amaterial having a high refractive index is filled in the space 113between the plurality of nanostructures 112

In order to reduce the physical period of the nanostructure 112, themanufacturing process may be difficult and the manufacturing cost mayincrease. According to the example embodiment, the planarization layer120 may be disposed on the upper surface of the nanostructure 112 suchthat the air gap exists between the plurality of nanostructures 112 ofthe reflective layer 110, and thus the refractive index of the space 113between the plurality of nanostructures 112 may remain low. Then,because the period of the plurality of nanostructures 112 may increase,the reflective layer 110 may be formed relatively easily, and thus, themanufacturing cost of the light emitting device 100 may be reduced.

In addition, the planarization layer 120 may be configured to haveconductivity. A driving circuit for controlling the operation of thelight emitting device 100 may provide a driving signal to the firstelectrode 131 through the reflective layer 110 having conductivity. Tothis end, the driving circuit may be connected to the reflective layer110. However, when the planarization layer 120 disposed between thereflective layer 110 and the first electrode 131 has insulatingproperty, a process of forming a via hole vertically in a partial regionof the planarization layer 120 and filling a conductive material in thevia hole may be added to electrically connect the reflective layer 110and the first electrode 131. Due to such an additional process, themanufacturing cost of the light emitting device 100 may increase and theperformance of the reflective layer 110 or the planarization layer 120may deteriorate. Therefore, the planarization layer 120 may be formed tohave conductivity, thereby reducing the manufacturing cost of thelight-emitting device 100 and preventing or reducing the performancedeterioration of the reflective layer 110 or the planarization layer120.

In addition, the planarization layer 120 may have a transparent propertywith respect to light generated from the organic emission layer 140. Forexample, the planarization layer 120 may include a transparent materialwith respect to visible light. Because the planarization layer 120 isdisposed in the micro cavity formed between the reflective layer 110 andthe second electrode 132, the planarization layer 120 may be formed toreduce light absorption by the planarization layer 120.

As described above, the planarization layer 120 may have conductivityand may be transparent to visible light. In the process of disposing theplanarization layer 120 on the reflective layer 110, the planarizationlayer 120 does not fill the space 113 between the plurality ofnanostructures 112 and is configured to maintain a flat surface statewith little deformation even on areas corresponding to the space 113between the plurality of nanostructures 112.

In general, it may not be easy to satisfy the above requirements of theplanarization layer 120 with a single material. For example, when ametal material is used as the planarization layer 120, the metalmaterial easily fills the space 113 between the plurality ofnanostructures 112 while forming the planarization layer 120 on thereflective layer 110. In addition, the use of the metal material causesa large loss of light. Meanwhile, graphene having a small thickness hasconductivity and light transmittance. However, when graphene having thesmall thickness is disposed on the reflective layer 110, because thegraphene is easily concavely dented toward the space 113 between theplurality of nanostructures 112, the first electrode 131, the organicemission layer 140, and the second electrode 132 disposed on thegraphene are difficult to maintain a flat state.

The planarization layer 120 according to the example embodiment mayinclude an organic-inorganic hybrid layer in which graphene, inparticular, reduced graphene oxide, is dispersed. For example, theorganic-inorganic hybrid layer of the planarization layer 120 mayinclude an organic silicon compound. Such an organic-inorganic hybridlayer may be transparent and flat and provide sufficiently highrigidity. In addition, the reduced graphene oxide dispersed in theorganic-inorganic hybrid layer may provide conductivity.

Hereinafter, a process of manufacturing the planarization layer 120according to the example embodiment will be described.

First, graphene oxide (GO) is dispersed in de-ionized (DI) water andions contained in the graphene oxide solution are removed throughdialysis. When compared with graphene oxide, graphene by itself is noteasily dispersed in water. FIG. 5 is a structural formula showing anexample of graphene oxide. As shown in FIG. 5 , graphene oxide has aconfiguration in which an oxygen atom is bonded to a carbon atom and/ora carboxyl group (—COOH) or a hydroxyl group (—OH) is further bonded toa carbon atom. Graphene oxide may be easily dispersed in water due tothe oxygen atom, the carboxyl group, or the hydroxyl group bonded to thecarbon atom of graphene oxide.

Then, a solution in which graphene oxide is dispersed and tetramethylorthosilicate (TMOS) sol are mixed together with a solvent to react. Forexample, the solvent may be ethanol. In addition, an acidic solutionsuch as hydrochloric acid (HCl) may be further mixed as a reactioncatalyst. The ratio of the solution in which graphene oxide isdispersed, the solvent, and the TMOS may be, for example, 1:1:0.1.

TMOS has a structure in which four oxygen atoms are bonded to onesilicon atom, and a methyl group (—CH3) is bonded to each oxygen atom.Such TMOS is hydrolyzed in a solvent to have a structure in which ahydroxyl group is bonded to each silicon atom, and hydrolyzed TMOS maybe bonded to the surface of graphene oxide. After reacting at roomtemperature for about 1 day, a mixture of graphene oxide and TMOS sol inthe solvent may be coated on a hydrophilic substrate. For example, themixture may be coated on a SiO₂/Si substrate using a spin coatingmethod.

Thereafter, the solvent in the mixture is removed by baking the mixturein a chamber. After removing the solvent, for example, the mixturecoated on the substrate is cured by heating the mixture at a temperatureof about 400° C. in the nitrogen (N₂) atmosphere. In this process, apolymer silica hybrid layer is formed through a condensation reaction.

Finally, graphene oxide is reduced to reduced graphene oxide byannealing the cured mixture, and thus the planarization layer 120 may beformed. For example, annealing may be performed at a temperature ofabout 1000° C. in the hydrogen (H₂) and nitrogen (N₂) atmosphere. Then,the oxygen atom, the carboxyl group, or the hydroxyl group bonded to thecarbon atom of graphene oxide shown in FIG. 5 may be removed.Accordingly, reduced graphene oxide having conductivity is dispersed anddistributed in the polymer silica hybrid layer. Therefore, theplanarization layer 120 may be the polymer silica hybrid layer in whichreduced graphene oxide is dispersed and distributed. The planarizationlayer 120 formed as described above may be removed from the hydrophilicsubstrate and transferred to and disposed on the reflective layer 110.

In order to confirm various characteristics of the planarization layer120 formed as described above, a plurality of planarization layers 120are formed by varying the concentration of graphene oxide. For example,FIG. 6 is a table showing an example of various combinations ofmaterials for manufacturing the planarization layer 120. As shown in thetable of FIG. 6 , the planarization layer 120 is actually formed byvariously selecting the proportion of graphene oxide compared to TMOS toabout 2 wt %, 4 wt %, 6 wt %, 8 wt %, and 10 wt % using theabove-described method.

The planarization layer 120 is formed to have a uniform thickness ofabout 40 nm as a whole. In the actual light emitting device 100, thethickness of the planarization layer 120 does not exceed about 50 nm.When the thickness of the planarization layer 120 is extremely great,the thickness of the organic emission layer 140 needs to be reduced,which may cause deterioration of the emission efficiency and brightnessof the light emitting device 100. For example, the thickness of theplanarization layer 120 may be in the range of about 10 nm to about 50nm.

FIGS. 7A to 7E are diagrams showing measurement results of the surfaceroughness of the planarization layer 120 according to the content ofgraphene oxide. FIGS. 7A to 7E illustrate that because when theproportion of graphene oxide compared to TMOS is 2 wt %, 4 wt %, 6 wt %,8 wt %, and 10 wt %, the root mean square (RMS) surface roughness of theplanarization layer 120 is about 0.434 nm, about 0.318 nm, about 0.407nm, about 0.343 nm, about 0.419 nm, respectively the surface roughnessof the formed planarization layer 120 is very high. Accordingly, theplanarization layer 120 according to the example embodiment may providea very flat surface to the first electrode 131, the organic emissionlayer 140, and the second electrode 132 disposed thereon. Although thesurface roughness of the planarization layer 120 may vary depending onthe thickness of the formed planarization layer 120, the surfaceroughness of the planarization layer 120 may be less than 1 nm RMS. Forexample, the surface roughness of the planarization layer 120 may be inthe range of about 0.3 nm RMS to about 0.5 nm RMS.

FIGS. 8A to 8F are scanning electron microscope (SEM) images showing theplanarization layer 120 transferred onto the reflective layer 110 whichare obtained with respect to various positions of the planarizationlayer 120 at various magnifications. The planarization layer 120 istransferred onto only a partial region of the reflective layer 110 tonot cover the entire region of the reflective layer 110. FIGS. 8A to 8Fillustrate that the planarization layer 120 transferred onto thereflective layer 110 provides a very flat surface. In addition, the edgeof the transferred planarization layer 120 that is the space 113 betweenthe plurality of nanostructures 112 of the reflective layer 110 is notfilled by the planarization layer 120 and has an air gap. In addition,the planarization layer 120 is not concavely recessed in the space 113between the plurality of nanostructures 112 of the reflective layer 110and maintains a flat surface state.

FIG. 9 is a graph showing the relationship between the wavelength ofincidence light and the transmittance of the planarization layer 120with respect to various contents of graphene oxide. In the graph of FIG.9 , the transmittance is relative to a glass substrate. For example, inthe graph of FIG. 9 , transmittance of 100% indicates that thetransmittance is the same as that of the glass substrate, andtransmittance of 80% indicates 80% of the transmittance of the glasssubstrate. FIG. 9 illustrates that the planarization layer 120 has arelatively low transmittance with respect to light in an ultraviolet rayregion and a relatively high transmittance with respect to light in avisible light region regardless of the content of graphene oxide. Inparticular, the lower the content of graphene oxide, the higher thetransmittance of the planarization layer 120. For example, when theproportion of graphene oxide compared to TMOS is about 2 wt %, thetransmittance of the planarization layer 120 is almost the same as thatof the glass substrate. In addition, even when the proportion ofgraphene oxide compared to TMOS is about 10 wt %, the transmittance ofthe planarization layer 120 may maintain 90% or more of thetransmittance of the glass substrate.

FIG. 10 is a graph showing changes in the electrical conductivity andthe transmittance of the planarization layer 120 according to thecontent of graphene oxide. Referring to FIG. 10 , as the proportion ofgraphene oxide compared to TMOS increases, the electrical conductivityof the planarization layer 120 increases, but the transmittance thereofdecreases. In addition, as the proportion of graphene oxide compared toTMOS decreases, the electrical conductivity of the planarization layer120 decreases, but the transmittance thereof increases.

FIG. 11 is a graph showing the current-voltage behavior of theplanarization layer 120 according to the content of graphene oxide. Thegraph of FIG. 11 illustrates that the planarization layer 120 exhibits avery linear current-voltage behavior regardless of the content ofgraphene oxide. In addition, as the content of graphene oxide increases,the resistance of the planarization layer 120 decreases and theelectrical conductivity thereof increases.

As described above, in all cases where the proportion of graphene oxidecompared to TMOS is 2 wt %, 4 wt %, 6 wt %, 8 wt %, and 10 wt %, theplanarization layer 120 has sufficient electrical conductivity andtransmittance. However, when the proportion of graphene oxide comparedto TMOS is too low, the electrical conductivity of the planarizationlayer 120 may be less than a reference value of the electricalconductivity, and when the proportion of graphene oxide compared to TMOSis too high, the transmittance of the planarization layer 120 may beless than a reference value of the transmittance. In addition, theproportion of reduced graphene oxide dispersed in the finally formedplanarization layer 120 may be directly determined by the proportion ofgraphene oxide compared to TMOS. In consideration of this point, theproportion of reduced graphene oxide dispersed in the planarizationlayer 120 may be selected within the range of about 1.5 wt % to about 15wt %.

FIG. 12 is a cross-sectional view schematically showing a structure of alight emitting device 200 according to another example embodiment.Referring to FIG. 12 , the light emitting device 200 according toanother example embodiment may include a reflective layer 210 includinga phase modulation surface, a planarization layer 220 disposed on thereflective layer 210, a first electrode 231 disposed on theplanarization layer 220, an organic emission layer 240 disposed on thefirst electrode 231, and a second electrode 232 disposed on the organicemission layer 240. In addition, the light emitting device 200 mayfurther include a transparent passivation layer 250 disposed on thesecond electrode 232. Compared with the light emitting device 100 shownin FIG. 1 , the structure of the phase modulation surface formed on thereflective layer 210 of the light emitting device 200 shown in FIG. 12is different from the structure of a phase modulation surface of thelight emitting device 100 shown in FIG. 1 . The remaining configurationof the light emitting device 200 illustrated in FIG. 12 is the same asthat of the light emitting device 100 illustrated in FIG. 1 , and thusdescriptions thereof will be omitted.

FIG. 13 is a perspective view schematically showing an example structureof the reflective layer 210 illustrated in FIG. 12 , and FIG. 14 is aplan view schematically showing an example structure of the reflectivelayer 210 illustrated in FIG. 12 . Referring to FIGS. 12 to 14 , thephase modulation surface may include a plurality of nanostructures 212protruding and periodically disposed on an upper surface 214 of a base211 facing the first electrode 231 and a plurality of recesses 213concaved in the upper surface 214 of the base 211. The upper surface ofthe plurality of nanostructures 212 may be in contact with theplanarization layer 220, and an air gap exists in a space 215 betweenthe plurality of nanostructures 212.

Each of the nanostructures 212 protruding from the upper surface 214 ofthe base 211 and each of the recesses 213 recessed from the uppersurface 214 of the base 211 may have dimensions smaller than thewavelength of visible light. As shown in FIGS. 13 and 14 , thenanostructures 212 and the recesses 213 may be formed to be spacedapart, and an area occupied by the upper surface 214 may be greater thanan area occupied by the plurality of nanostructures 212 or the pluralityof recesses 213. In addition, the area occupied by each of thenanostructures 212 may be greater than or equal to the area occupied byeach of the recesses 213.

The plurality of nanostructures 212 may be periodically arranged with apredetermined pitch P1 on the upper surface 214 of the base 211. FIG. 14shows an example of the nanostructures 212 periodically arranged in theshape of a square array. However, this is merely an example, and inaddition, the plurality of nanostructures 212 may be arranged in anarray of various other shapes such as a regular triangle, a regularhexagon, etc. Each of the nanostructures 212 may have, for example, adiameter W1 of about 300 nm or less. However, each of the nanostructures212 is not necessarily limited thereto. For example, each of thenanostructures 212 may have the diameter W1 of about 30 nm to 250 nm.Further, each of the nanostructures 212 may have, for example, a heightH1 of about 100 nm or less. However, these numerical values are onlyexamples, and embodiments are not limited thereto.

As described above, the plurality of nanostructures 212 may serve toadjust the optical length L of the micro cavity to resonate lightcorresponding to the emitting wavelength of the light emitting device200. In other words, when the resonance wavelength of the micro cavityis λ, the diameter W1 and the height H1 of each of the nanostructures212 of the phase modulation surface and the pitch P1 of thenanostructures 212 may be selected such that the optical length L of themicro cavity satisfies nλ/2, where n is a natural number.

The plurality of recesses 213 may be formed at a predetermined depth H2on the upper surface 214 of the base 211. The plurality of recesses 213may be periodically two-dimensionally arranged with a predeterminedpitch P2 between the plurality of nanostructures 212. FIGS. 13 and 14show examples of each of the recesses 213 disposed between the twoadjacent nanostructures 212. Each of the recesses 213 may be formed in acylindrical shape. Each of the recesses 213 may have, for example, adiameter W2 of approximately 250 nm or less. More specifically, forexample, each of the recesses 213 may have a diameter W2 of about 80 nmto 250 nm, but is not limited thereto. Further, each of the recesses 213may have, for example, a depth H2 of about 100 nm or less but this ismerely an example. In addition, a difference between the diameter W1 ofeach of the nanostructures 212 and the diameter W2 of each of therecesses 213 may be, for example, about 100 nm or less, but is notlimited thereto.

The plurality of the recesses 213 may serve to absorb light of awavelength of which resonance is not desired within the micro cavity.FIG. 15A schematically shows light of a short wavelength flowing intothe recess 213 formed in the reflective layer 210, and FIG. 15Bschematically shows light of a long wavelength blocked in the reflectivelayer 210 in which the recess 213 is formed. It may be seen that asshown in FIG. 15A, the light of the short wavelength flows into and isabsorbed in the nano-sized recess 213 formed in the upper surface 214 ofthe base 211, whereas, as shown in FIG. 15B, the light of the longwavelength does not flow into the recess 213 and is reflected from theupper surface 214 of the base 211.

The wavelength of the light absorbed into the recess 213 formed in thereflective layer 210 may vary according to the size of the recess 213.For example, when the nanostructures 212 are not considered, the recess213 having a diameter of about 190 nm formed on the surface of the flatreflective layer 210 including silver (Ag) may absorb blue light of awavelength of 450 nm, and the recess 213 having a diameter of about 244nm may absorb green light of a wavelength of 550 nm.

For example, in the light emitting device 200 configured to emit redlight, when the optical length L of the micro cavity is selected as 630nm, part of light of a wavelength of 420 nm may cause a third resonanceto be emitted from the light emitting device 200. Then, because bluelight is emitted from the light emitting device 200 together with redlight, the color purity of light emitted from the light emitting device200 may be reduced. In the example embodiment, the light of thewavelength of which resonance is not desired may be additionallyabsorbed by the recess 213 by forming the plurality of nano-sizedrecesses 213 along with the plurality of nanostructures 212 on the phasemodulation surface of the reflective layer 210. Therefore, the colorpurity of the light emitting device 200 may be further improved.

FIG. 16 schematically shows light resonating in the light emittingdevice 200 according to an example embodiment. In FIG. 16 , a red lightemitting device is illustrated as the light emitting device 200 as anexample, and for convenience, only the reflective layer 210 and thesecond electrode 232 constituting the micro cavity are illustrated.Referring to FIG. 16 , in the micro cavity, a red light R may not flowinto the recess 213 formed in the surface of the reflective layer 210but may be reflected from the surface of the reflective layer 210.However, a blue light B having a wavelength shorter than the red light Rflows into and is absorbed in the recess 213 formed in the surface ofthe reflective layer 210. As described above, each of the recesses 213may have, for example, a diameter of about 250 nm or less. Accordingly,in the micro cavity, only the red light R may resonate and be emittedoutside the light emitting device 200.

In FIG. 16 , the example in which the light emitting device 200according to the example embodiment is the red light emitting device isdescribed. However, embodiments are not limited thereto. For example,the light emitting device 200 according to the example embodiment may bea green light emitting device. In general, in a case where the surfaceof a reflective layer has a flat structure, when a second resonance of agreen light occurs in a micro cavity, a third resonance of anultraviolet light occurs, which does not affect a display apparatus in avisible light region. However, when the reflective layer 210 having thephase modulation surface is used, there is a possibility that a thirdresonance of the blue light B occurs in the micro cavity due to thephase modulation. Accordingly, blue light of an unwanted shortwavelength in the green light emitting device may be emitted. Therefore,even when the light emitting device 200 is the green light emittingdevice, the unwanted emission of the blue light may be suppressed byforming the plurality of recesses 213 in the surface of the reflectivelayer 210.

FIG. 17 is a plan view schematically showing another example structureof the reflective layer 210 shown in FIG. 12 . In the example embodimentshown in FIGS. 13 and 14 , the nanostructures 212 are periodicallyarranged in a square array, and each of the recesses 213 may be formedbetween the two adjacent nanostructures 212. In the reflective layer 210shown in FIG. 17 , the nanostructures 212 protruding from the uppersurface 214 of the base 211 may be periodically arranged in the squarearray, and the recesses 213 may be arranged between the twonanostructures 212 arranged adjacent to each other in a diagonaldirection on the upper surface 214 of the base 211 at a predetermineddepth. For example, each of the recesses 213 may be disposed in thecenter of a unit array of a square shape including the four adjacentnanostructures 212. However, this is merely an example, and thenanostructures 212 and the recesses 213 may be arranged in various othershapes.

In addition, FIG. 18 is a perspective view schematically showing anotherexample structure of the reflective layer 210 shown in FIG. 12 . In theexample embodiment shown in FIGS. 13 and 14 , the nanostructures 212have a cylindrical shape and the recesses 213 are formed in acylindrical shape. In the metal reflective layer 210 shown in FIG. 18 ,the nanostructures 212 have a square column shape. In this example, themaximum width of the nanostructure 212 may correspond to the diameter.In addition, the recesses 213 may be formed in the cylindrical shapebetween the two adjacent nanostructures 212. However, this is merely anexample, and each of the nanostructures 212 may have a variety of otherpolyprism shapes, such as a triangular column or a pentagonal column. Inaddition, each of the recesses 213 may also be formed in various othershape.

The above-described light emitting devices 100 and 200 may be applied toa plurality of pixels of a display apparatus. FIG. 19 is across-sectional view schematically showing a structure of a displayapparatus 1000 according to an example embodiment. Referring to FIG. 19, the display apparatus 1000 may include a plurality of pixels that emitlight of different colors. Here, the plurality of pixels may includered, green, and blue pixels 1100, 1200, and 1300 disposed adjacent toeach other on the same plane of a substrate. As an example, only oneunit pixel including the red, green, and blue pixels 1100, 1200, and1300 is illustrated.

The red pixel 1100 may have the same structure as the light emittingdevice 100 illustrated in FIG. 1 . The red pixel 1100 may include afirst reflective layer 1110 including a first phase modulation surface,a first planarization layer 1120 disposed on the first reflective layer1110, a first electrode 1131 disposed on the first planarization layer1120, an organic emission layer 1140 disposed on the first electrode1131, and a second electrode 1132 disposed on the organic emission layer1140. The red pixel 1100 may further include a transparent passivationlayer 1150 disposed on the second electrode 1132. The first reflectivelayer 1110 may include a plurality of first nanostructures 1112 formedto protrude on an upper surface 1114 of a base 1111. The firstreflective layer 1110 may form a first micro cavity that resonates thered light R together with the second electrode 1132. In addition, uppersurfaces of the plurality of first nanostructures 1112 may be in contactwith the planarization layer 1120, and an air gap exists in a space 1115between the plurality of first nanostructures 1112.

The green pixel 1200 may have the same structure as the light emittingdevice 100 shown in FIG. 1 . The green pixel 1200 may include a secondreflective layer 1210 including a second phase modulation surface, asecond planarization layer 1220 disposed on the second reflective layer1210, the first electrode 1131 disposed on the second planarizationlayer 1220, the organic emission layer 1140 disposed on the firstelectrode 1131, the second electrode 1132 disposed on the organicemission layer 1140, and the passivation layer 1150 disposed on thesecond electrode 1132. The second reflective layer 1210 may include aplurality of second nanostructures 1212 formed to protrude over an uppersurface 1214 of a base 1211. The second reflective layer 1210 may form asecond micro cavity that resonates the green light G together with thesecond electrode 1132. In addition, upper surfaces of the plurality ofsecond nanostructures 1212 may be in contact with the planarizationlayer 1120, and an air gap exists in a space 1215 between the pluralityof second nanostructures 1212.

In addition, the blue pixel 1300 may include a third reflective layer1310, a third planarization layer 1320 disposed on the third reflectivelayer 1310, a first electrode 1131 disposed on the third planarizationlayer 1320, the organic emission layer 1140 disposed on the firstelectrode 1131, the second electrode 1132 disposed on the organicemission layer 1140, and the passivation layer 1150 disposed on thesecond electrode 1132. The upper surface of the third reflective layer1310 in the blue pixel 1300 may include a flat reflective surface. Thethird reflective layer 1310 may form a third micro cavity that resonatesblue light together with the second electrode 1132. The third microcavity may have a resonance wavelength of the blue light B by adjustingstructural and optical characteristics of the layers disposed betweenthe third reflective layer 1310 and the second electrode 1132. Here, theupper surface of the third reflective layer 1310 may be formed at thesame height as the upper surfaces of the first and second nanostructures1112 and 1212.

FIG. 20 is a cross-sectional view schematically showing a structure of adisplay apparatus 2000 according to another example embodiment.Referring to FIG. 20 , the first reflective layer 1110 of the red pixel1100 of the display apparatus 2000 may further include a plurality offirst recesses 1113 that absorb the blue light B. Although the secondreflective layer 1210 of the green pixel 1200 includes only the secondnanostructure 1212, the second reflective layer 1210 of the green pixel1200 also may further include a plurality of second recesses forabsorbing blue light B.

In the display apparatuses 1000 and 2000 illustrated in FIGS. 19 and 20, the planarization layer 1120 forms an air gap in the space 1115between the plurality of first nanostructures 1112 and the space 1215between the plurality of second nanostructures 1212, there is no need tomanufacture the first and second nanostructures 1112 and 1212 with aperiod of 100 nm or less. Accordingly, manufacturing of the displayapparatuses 1000 and 2000 may be more easy and manufacturing cost may bereduced.

The above-described light emitting device and display device may beapplied without limitation to devices of various sizes and various uses.For example, the above-described light emitting device and displayapparatus may be applied to a display panel of a mobile phone or a smartphone, may be applied to a display panel of a tablet or smart tablet,may be applied to a display panel of a notebook computer, television, orsmart television, or may be applied to a small display panel used in ahead mounted display, a glasses type display, a goggle type display, orthe like.

While the light emitting device, the method of manufacturing the lightemitting device and the display apparatus including the light emittingdevice are described according to example embodiments with reference tothe accompanying drawings, it should be understood that exampleembodiments described herein should be considered in a descriptive senseonly and not for purposes of limitation. Descriptions of features oraspects within each example embodiment should typically be considered asavailable for other similar features or aspects in other embodiments.While one or more example embodiments have been described with referenceto the figures, it will be understood by those of ordinary skill in theart that various changes in form and details may be made therein withoutdeparting from the spirit and scope as defined by the following claims.

What is claimed is:
 1. A light emitting device comprising: a reflectivelayer comprising a plurality of nanostructures that are periodicallytwo-dimensionally arranged; a planarization layer disposed on thereflective layer; a first electrode disposed on the planarization layer;an organic emission layer disposed on the first electrode; and a secondelectrode disposed on the organic emission layer, wherein theplanarization layer comprises a conductive material that is transparentwith respect to light emitted by the organic emission layer, wherein theplanarization layer is disposed on upper surfaces of the plurality ofnanostructures such that an air gap is provided between adjacentnanostructures of the plurality of nanostructures, and wherein theplanarization layer further comprises an organic-inorganic hybrid layerin which reduced graphene oxide is dispersed.
 2. The light emittingdevice of claim 1, wherein the organic-inorganic hybrid layer comprisesan organic silicon compound.
 3. The light emitting device of claim 1,wherein a proportion of the reduced graphene oxide dispersed in theplanarization layer is in a range of 1.5 wt % to 15 wt %.
 4. The lightemitting device of claim 1, wherein a thickness of the planarizationlayer is in a range of 10 nm to 50 nm.
 5. The light emitting device ofclaim 4, wherein a surface roughness of the planarization layer is lessthan 1 nm root-mean-square (RMS).
 6. The light emitting device of claim5, wherein the surface roughness of the planarization layer is in arange of 0.3 nm RMS to 0.5 nm RMS.
 7. The light emitting device of claim1, wherein the first electrode is a transparent electrode and the secondelectrode is a semi-transmissive electrode that is configured to reflectpart of light and transmit a remaining part of the light.
 8. The lightemitting device of claim 7, wherein the second electrode comprises areflective metal, and a thickness of the second electrode is in a rangeof 10 nm to 50 nm.
 9. The light emitting device of claim 1, wherein thereflective layer and the second electrode form a micro cavity having aresonance wavelength.
 10. The light emitting device of claim 9, whereinthe resonance wavelength of the micro cavity is λ, a diameter of each ofthe plurality of nanostructures of the reflective layer, a height ofeach of the plurality of nanostructures, and a period of the pluralityof nanostructures are determined such that an optical length of themicro cavity satisfies nλ/2, where n is a natural number.
 11. The lightemitting device of claim 9, wherein a period of the plurality ofnanostructures is less than the resonance wavelength of the microcavity.
 12. The light emitting device of claim 1, wherein the reflectivelayer further comprises a base, and wherein the plurality ofnanostructures protrude toward the planarization layer from an uppersurface of the base.
 13. The light emitting device of claim 12, whereinthe reflective layer further comprises a plurality of recesses that arerecessed from the upper surface of the base and two-dimensionallyprovided.
 14. The light emitting device of claim 13, wherein thereflective layer and the second electrode form a micro cavity having aresonance wavelength, wherein the organic emission layer is configuredto emit visible light comprising light of a first wavelength and lightof a second wavelength, wherein the first wavelength is λ, a diameter ofeach of the plurality of nanostructures of the reflective layer, aheight of each of the plurality of nanostructures, and a period of theplurality of nanostructures are determined such that an optical lengthof the micro cavity satisfies nλ/2, where n is a natural number, andwherein a diameter of each of the plurality of recesses is determinedsuch that the plurality of recesses are configured to absorb the lightof the second wavelength.
 15. The light emitting device of claim 1,wherein the reflective layer comprises a metal material comprisingsilver (Ag), aluminum (Al), gold (Au), nickel (Ni), or an alloy thereof.16. A display apparatus comprising: a first pixel configured to emitlight of a first wavelength; and a second pixel configured to emit lightof a second wavelength different from the first wavelength, wherein thefirst pixel comprises: a reflective layer comprising a plurality ofnanostructures that are periodically two-dimensionally arranged; aplanarization layer disposed on the reflective layer; a first electrodedisposed on the planarization layer; an organic emission layer disposedon the first electrode, the organic emission layer being configured toemit visible light comprising the light of the first wavelength and thelight of the second wavelength; and a second electrode disposed on theorganic emission layer, wherein the planarization layer comprises aconductive material that is transparent with respect to light emitted bythe organic emission layer, and wherein the planarization layer isdisposed on upper surfaces of the plurality of nanostructures such thatan air gap is provided between adjacent nanostructures of the pluralityof nanostructures.
 17. The display apparatus of claim 16, wherein theplanarization layer further comprises an organic-inorganic hybrid layerin which reduced graphene oxide is dispersed.
 18. The display apparatusof claim 17, wherein the organic-inorganic hybrid layer comprises anorganic silicon compound.
 19. The display apparatus of claim 17, whereina proportion of the reduced graphene oxide dispersed in theplanarization layer is in a range of 1.5 wt % to 15 wt %.
 20. Thedisplay apparatus of claim 16, wherein a thickness of the planarizationlayer is in a range of 10 nm to 50 nm.
 21. The display apparatus ofclaim 20, wherein a surface roughness of the planarization layer is lessthan 1 nm root-mean-square (RMS).
 22. The display apparatus of claim 21,wherein the surface roughness of the planarization layer is in a rangeof 0.3 nm RMS to 0.5 nm RMS.
 23. The display apparatus of claim 16,wherein the first electrode is a transparent electrode and the secondelectrode is a semi-transmissive electrode that is configured to reflectpart of light and transmit a remaining part of the light.
 24. Thedisplay apparatus of claim 16, wherein the reflective layer and thesecond electrode form a micro cavity having a resonance wavelengthcorresponding to the first wavelength, and wherein the first wavelengthis λ, a diameter of each of the plurality of nanostructures of thereflective layer, a height of each of the plurality of nanostructures,and a period of the plurality of nanostructures are determined such thatan optical length of the micro cavity satisfies nλ/2, where n is anatural number.
 25. A method of manufacturing a light emitting device,the method comprising: coating a substrate with a mixture of grapheneoxide and tetramethyl orthosilicate (TMOS) sol in a solvent; curing themixture coated on the substrate; annealing the cured mixture to reducegraphene oxide to reduced graphene oxide and form a planarization layercomprising an organic-inorganic hybrid layer in which the reducedgraphene oxide is dispersed; transferring the planarization layer to areflective layer comprising a plurality of nanostructures that areperiodically two-dimensionally arranged; disposing a first electrode onthe planarization layer; disposing an organic emission layer on thefirst electrode; and disposing a second electrode on the organicemission layer, wherein the planarization layer is conductive andtransparent with respect to light emitted by the organic emission layer,wherein the planarization layer is disposed on upper surfaces of theplurality of nanostructures such that an air gap is provided betweenadjacent nanostructures the plurality of nanostructures.
 26. A lightemitting device comprising: a reflective layer comprising: a pluralityof nanostructures that are periodically two-dimensionally provided; anda plurality of recesses that are periodically two-dimensionallyprovided; a planarization layer disposed on the reflective layer; afirst electrode disposed on the planarization layer; an organic emissionlayer disposed on the first electrode; and a second electrode disposedon the organic emission layer, wherein the planarization layer comprisesa conductive material transparent with respect to light emitted by theorganic emission layer, wherein the planarization layer is disposed onupper surfaces of the plurality of nanostructures such that an air gapis provided between adjacent nanostructures of the plurality ofnanostructures, and wherein the planarization layer further comprises anorganic-inorganic hybrid layer in which reduced graphene oxide isdispersed.